Gridded electron gun

ABSTRACT

The control grid structure for a convergent flow electron gun includes a corrective electrostatic lens associated with each of the beam passageways through the grid. The corrective electrostatic lenses increase in focusing strength taken from the outside of the beam toward the center of the beam to compensate for beamlet defocusing forces occasioned by the relatively large central aperture in the anode of the gun. In a preferred embodiment, the corrective electrostatic focusing lens structure comprises a second control grid portion interposed between a first control grid portion and the anode. The second control grid portion is thermally shielded from the cathode via the first grid portion for inhibiting unwanted thermionic emission from the composite control grid structure.

O United States Patent 1 1 3,852,633

Hunter Dec. 3, 1974 GRIDDED ELECTRON GUN Primary Examiner.lames W.Lawrence Assistant ExaminerSaxfield Chatmon, Jr. [75] Inventor' 2252 T.Hunter Mountam View Attorney, Agent, or Firm-R. K. Stoddard; H. E. Aine[73] Assignee: Varian Associates, Palo Alto, Calif. 22 Filed: Dec. 13,1972 [57] ABSTRACT [21] APPL No: 314,660 The control grid structure fora convergent flow elec- 4 tron gun includes a corrective electrostaticlens associv ated with each of the beam passageways through the [5. UQS':1. 1314 8; grid. The corrective electrostatic lenses increase infocusing strength taken from the outside of the beam to- Int. Cl. wardthe center of the beam to compensate for beam- Field of Search letdefocusing forces occasioned by the relatively large 313/343, 82 R, 82BF central aperture in the anode of the gun. in a preferred embodiment,the corrective electrostatic focusl l References Cited ing lensstructure comprises a second control grid por- UNITED STATES PATENTStion interposed between a first control grid portion 3,377,492 4/l968Oess s. 313/348 x and the anode- The Second Control grid Pomon is3,484,645 9/1971 Drees r 313/348 thermally Shielded from the Cathode viathe fi grid 3,500,110 3/1970 Winsor 3l5/3.5 portion for inhibitingunwanted thermionic emission 3,558,967 6/1969 Miriam BIS/3.5 from thecomposite control grid'structure. 3,651,360 3/1972 Sommeria 313/82 R I 6Claims, 10 Drawing Figures Pmmw 31m 3.852.633

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PATENIELUEB 31914 3.852.633 sum 3 or 3 PRIMARY CONTROL PRIMARY CONTROL'ELECTRODE WITH ELECTRODE WITH EMISSION EMISSION I2 '2" SECONDARYCONTROL H98 5 I ELECTRODE GRIDDED ELECTRON GUN BACKGROUND OF THEINVENTION The present invention relates in general to gridded convergentflow electron guns and, more particularly, to an improved control gridstructure for such guns which reduces undesired beam aberration, allowsincreased beam convergence, and which reduces unwanted interpulse noisecaused by undesired thermionic emission from the control grid structure.

BRIEF DESCRIPTION OF THE PRIOR ART Heretofore, gridded convergent flowelectron guns have been built wherein the control grid comprised amulti-apertured spherically concave cathode emitting surface of athermionic cathode emitter for controlling the convergent flow ofelectrons from the concave cathode emitting surface through a centralaperture in an anode structure. The control grid comprised a multitudeof grid openings or beam passageways of uniform cross-sectionaldimensions for producing a multiplicity of individual beamlets whichpassed through the control grid in a substantially non-interceptingmanner and thence into a confluent flow of electrons through the centralaperture in the anode to produce a pencil-like cylindrical beam ofelectrons. In a preferred embodiment, the spherically concave cathodeemitting surface was dimpled with a multiplicity of lesser sphericallyconcave cathode emitting surfaces forming the composite sphericallyconcave cathode emitting surface. The lesser concave cathode emittingsurfaces were each aligned along the beam path with the respectiveopenings in the control grid so as to minimize interception of currenton the control grid structure. Such a gridded electron gun is disclosedand claimed in US.

Pat. No. 3,558,967 issued Jan. 26, 1971 and assigned to the sameassignee as the present invention.

One of the problems with this prior art control grid structure is thatthe relatively large central aperture in the anode produces a distortionin the equipotential lines adjacent the central region of the controlgrid structure. This distortion of equipotential lines adja-' cent thecontrol grid structure causes the central portion of the control gridstructure to act as a defocusing lens producing a substantial defocusingof the individual beamlets near the center of the control gridstructure. This defocusing force produces a substantial radial velocityto the electrons in the beamlets passing through the central portion ofthe control grid and therefore reduces the convergence of the compositebeam. Heretofore, attempts have been made to improve the convergence ofthe beam by extending an annular nose portion at the lip of the centralopening in the anode structure toward the center region of the controlgrid but this has a deleterious effect of reducing the high voltageholdoff potential that can be established between the control grid andanode.

In addition, another problem encountered in the prior art control gridstructure is that the control grid is closely spaced to the thermioniccathode emitter such that the control grid is heated by radiation. Ifthe temperature of the control grid becomes excessive, thermionicemission is obtained from the control grid which increases theinterpulse noise, i.e., grid emission produces an output when the gridis pulsed negative for turning off the beam.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved gridded electron gun.

In one feature of the present invention, an electrostatic beam focusingstructure is associated with the multi-apertured control grid forproviding an increasing beamlet focusing force toward the center of thecontrol grid structure to compensate for distorted equipotential linesbetween the control grid and the centrally apertured anode of theelectron gun.

In another feature of the present invention, a second control gridstructure is associated with the primary control grid, such secondarycontrol grid structure being interposed between the primary control gridand the anode. The secondary control grid is thermally shielded from thethermionic cathode emitter by the primary control grid and thus operatesat a low temperature for reducing undesired thermionic emission from thecontrol grid structure during periods when the control grid is pulsednegative for turning off the beam current.

In another feature of the present invention, the length of theindividual beam passageways through the control grid structure increasestoward the center of the control grid for providing an increasingfocusing force on the individual beamlets taken in the direction towardthe center of the control grid structure for improving the beamconvergence of the composite beam, and for increasing the standoffvoltage between the control grid and anode by allowing the anode to bemoved further from the control grid structure for a given beamconvergence.

Other features and advantages of the present inven tion will becomeapparent upon a perusal of the following specification taken inconnection with theaccompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinalsectional view, I

. :FIG. 4 is an enlarged detail view of a portion of the structure ofFIG. 1 delineated by line 4-4 and showing the equipotential lines in theelectron gun assembly,

FIG. 5 is an enlarged detail view of a portion of the structure of FIG.4 delineated by line 5-5 and depicting the equipotential lines for acontrol grid beamlet near the center of the composite cathode emitterusing only a single control grid of the prior art,

FIG. 6 is a view similar to that of FIG. 5 depicting the pattern ofequipotential lines and beam trajectories for a control grid beamletnear the center of the control grid structure of the present invention,

FIG. 7 is a longitudinal sectional view of a control grid structureincorporating features of the present invention,

FIG. 8 is a schematic longitudinal sectional view of a control gridbeamlet of a prior art electron gun showing thermionic emission from thecontrol grid structure,

FIG. 9 is a view similar to that of FIG. 8 depicting how the controlgrid structure of the present invention inhibits thermionic emissionfrom the control grid structure, and

FIG. is a view similar to that of FIG. 7 depicting an alternativecontrol grid structure of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a gridded convergent flow electron gun I incorporating featuresof the present invention. The gun 1 includes a thermionic cathodeemitter 2, as of the dispenser or oxide coated nickel type, having aspherically concave cathode emitting'surface 3 for supplying copiouselectron emission when heated to its operating temperature via a heatingelement 4 disposed in heat exchanging rela tion with the cathode 2. Afilament power supply 5 supplies electrical power to the heating element4. A centrally apertured anode disc 6, as of copper, is axially spacedalong an axis of revolution 7 from the concave cathode emitting surface3. The central aperture 8 in the anode has a flared entrance portion 9and a constricted neck portion having a transverse crosssectional areasubstantially smaller than the transverse cross-sectional area of theemitter 3 such that a substantial convergent flow of electrons isobtained from the cathode emitter 3 through the central aperture 8 inthe anode 6 when a suitable positive potential is applied to the anode 6relative to the cathode emitter 2 via anode supply 11.

A spherically concave multi-apertured control grid 12, as of molybdenumor tungsten, is disposed closely overlaying the concave cathode emittingsurface 3 for controlling the flow of electrons from the cathodeemitting surface 3 through the anode 6. The control grid 12 is supportedat its outer periphery via a cylindrical support structure 13 disposedin thermally and electrically insulative relation relative to thecathode 2. A grid bias supply 14 supplies a DC negative grid bias to thecontrol grid 12 so that the control grid is normally biased off, i.e.,the beam current is off. A grid pulser 15 is series connected withsupply 14 for applying a positive pulse of sufficient magnitude suchthat the control grid 12 is pulsed positive relative to the cathode 2for pulsing the beam current through the anode. A beam focus electrode16, typically operated at cathode potential, is provided surrounding thecontrol grid 12 to aid in focusing the electron beam through the centralaperture in the anode 6 in a substantially non-intercepting manner.

A more detailed description of the electron gun I and tubes using sameis found in the disclosure of the aforecited US. Pat. No. 3,558,967,which is hereby incorporated by reference.

Referring now to FIG. 2 there is shown the control grid hole pattern.More particularly, the holes are arranged in the sphericallyconcave'surface of the control grid structure in a plurality ofconcentric circular rows. One hole is preferably centered in the centerof the grid 12 on the axis of revolution 7 of both the control grid andthe underlying cathode emitter. The outer periphcry 18 of the underlyingcathode emitter 3 determines the outer peripheral boundary for the holepattern. This defines a spherical peripheral line P extending from theaxis of revolution 7 along the concave surface of the control grid tothe perimeter 18. Peripheral line I can accommodate so many circularrows of holes of a given size without the holes intersecting each other,i.e., the holes being a little less than tangent along the sphericalline P. Once the number of rows and hole size has been decided, theholes of this size are merely equally spaced around each of theconcentric rows beginning from the starting line P. The result is acircular array of closely spaced holes with an overall beam transparencyof between 65 and 75%. Such a control grid is disclosed and claimed incopending application Ser. No. 293,205 filed Sept. 28, 1972, nowabandoned, and assigned to the same assignee as the present invention.

Referring now to FIG. 3 there is shown the control grid 12 closelyspaced to the spherically concave emitting surface 3 of the cathodeemitter 2. In a typical example, the spacing from the cathode emittersurface 3 to the convex surface of the control grid 12 is approximately0.039 inches. The spherically concave control grid 12 has a radius ofcurvature slightly less than that of the concave cathode emittingsurface 3. Both the emitter 3 and grid 12 have a common center ofcurvature such that uniform spacing is obtained between the emittingsurface 3 and the control grid 12.

The composite concave cathode emitting surface 3 is formed by an arrayof spherically concave dimples 21 arranged in a circular pattern ofuniform size corresponding to the similar hole pattern of control grid12, such that the centers of the individual dimples 21 are axiallyaligned along the beam path with the centers of the registered holes inthe control grid 12. The dimples 21 have a radius of curvature which issubstantially less than that of the composite surface 3 to define, withthe corresponding grid holes, a multiplicity of convergent flow electronguns to generate a multiplicity of nonintercepting control grid electronbeamlets 22 for projecting thegrid controlled electron beam through thecontrol grid 12 in a non-intercepting manner. The beamlets converge intoa unitary beam after passage through the control grid.

A shadow grid 23, preferably of nickel in the case of a nickel oxidecoated cathode, has a hole pattern corresponding to the hole pattern inthe control grid 12 and has a radius of curvature equal to that of theemitting surface 3. The shadow grid 23 is disposed in nominal contactwith the web portion of the emitting surface 3 to inhibit emission fromthat portion of the cathode emitter surface 3 which is'opposite theindividual web portions of the control grid 12, whereby undesired gridinterception is prevented.

The control grid 12 includes a primary control grid portion 12. and asecondary control grid portion 12" axially spaced apart along thedirection of the beam to define a multitude ofcontrol grid beamletpassageways through the aligned openings in the composite grid structure12. The secondary control grid portion 12" has a radius of curvaturewhich is larger than the radius of curvature of the first or primarycontrol grid member 12' such that the spacing along the direction of thebeam path between the primary and secondary control grid portion 12' and12 increases toward the center of the composite control grid structure12. This increase in the length of the beam passageways through theindividual aligned openings in the control grid structure 12 toward thecenter of the composite grid structure 12 serves to produceelectrostatic beam focusing lenses of increasing magnitude taken in thedirection toward the center of the composite grid structure from itsouter periphery. This is for the purpose of compensating for anotherwise increasing defocusing electrostatic lens effect caused by theequipotentials moving away from the surface of the control gridstructure near the central portion of the control grid, as more clearlyshown in FIG. 4.

Referring now to FIG. 4, there is shown the pattern of electrostaticequipotentials 25 in the region between the cathode 2 and anode 6. Ascan be seen from the plot of FIG. 4, the equipotential lines bow awayfrom the center of the control grid 12 toward the central opening 6 inthe anode. When the equipotentials bow away from the plane of thecontrol grid 12, the equipotential lines at the control grid potentialtend to bow away from the cathode 2 at the beam entrance apertures ofthe control grid structure 12. This produces an electrostatic defocusingforce on the electrons of the individual beamlets passing through thegrid structure 12. This defocusing force tends to increase in magnitudetoward the center of the control grid and that portion of the beam whichoriginates from the central region of the cathode emitter2 has impartedthereto a substantial transverse velocity thereby reducing the overallconvergence of the electron beam.

In the prior art, wherein only one very thin control grid portion 12'was utilized for controlling the beam (see FIG. 5), attempts were madeto counteract this beam divergent effect near the center of the beam bymoving the annular anode nose portion closer into the center of thecontrol grid and cathode 2, as indicated by dotted line 26 of FIG. 4.While moving the nose portion of the anode in closer to the controlgrid'12 and toward the center of the control grid 12' served to improvethe shape of the equipotentialsand to decrease the divergence of thebeamlets near the .center of the control grid, it also had the undesiredeffect of decreasing the spacing between the focusing electrode 16 andanode 6 such that the maximum hold off voltage (beam voltage) betweenthe focusing electrode and the anode was substantially reduced. Sincethemaximum hold off voltage determines the maximum beam voltage of theelectron gun it is desirable to provide means for compensating for thebowed equipotential lines in such a manner that the maximum beam voltageis not deleteriously affected.

Referring again to FIG. 5, there is shown the plot of equipotentialsinthe vicinity of the primary control grid portion '12 in the case of theprior art control grid structure which included only one control gridportion 12. The plot of FIG. 5 is for a beamlet generated near I controlgrid portion 12 and that of the secondary con-' beamlet passagewaythrough the control grid 12 is produced by the secondary control gridportion 12" and produces a substantial corrective focusing force uponthe electrons of the beamlet, thereby compensating for the divergentlens effect produced by the primary control grid portion 12'. Thus, bythe provision of the secondary control grid portion 12", the divergingeffect of bowed equipotential lines near the center of the control gridstructure is substantially compensated to increase the convergence ofthe electron beam and to reduce radial aberrations, thereby increasingthe laminarity of the resultant electron stream. Increased convergenceallows the minimum beam diameter to be decreased. This result isachieved without deleteriously affecting the voltage hold off of theelectron gun I.

' Another advantage of the control grid 12 employing first and secondcontrol grid portions 12' and 12" is that the thermionic emission fromthe primary control grid portion 12 is intercepted by the web of thesecondary control grid portion 12". In addition the secondary controlgrid portion 12" is thermally shielded from the .high temperaturecathode emitter 2 by the primary control grid portion 12' such that thesecondary control electrode portion 12" operates at a'lower temperatureand thus thermionic emission from'the control grid structure issubstantially reduced. This greatly reduces interpulse noise. Thiscontrol of thethermionic grid emission is depicted in FIGS. 8 and 9where FIG. 8 shows the prior art grid having thermionic grid emissionand FIG. 9 shows the structure of the present invention employing theaxially spaced primary and secondary control grid portions 12' and 12".

' Referring now to FIG. 7v there is shown the control grid structure 12of the present invention. More particularly it-is seen that the spacingbetween the primary trol grid portion 12" increases toward the center ofthe control grid 12 for increasing the strength of the electrostaticfocusing lens action of the secondary control grid portion 12" towardthe center of the control grid the control grid structure to produce theincreasing the center of the control grid structure such that theoutward bowing of equipotential lines in the vicinity of the controlgrid produces a substantial defocusing or radial velocity component tothe electrons of the beamlet passing through the control grid 12'.

Referring now to FIG. 6 there is shown a plot similar to that of FIG. 5for the control grid structure of FIG. 3 having axially spaced gridportions 12 and 12". As

can be seen from the plot of the equipotential lines 25, the inwardlydished equipotential line 25' at the beam entrance aperture of theprimary control grid 12' is compensated for by an oppositely dishedequipotential line 25" produced at the exit of the beamlet passagewaythrough the control grid electrode 12. This inwardly dishedequipotential line 25" at the exit of the I corrective beam focusinglens effect at the beam exit ends of the beamlet passageways through thegrid structure to compensate for the outward bowing of the equipotentiallines near the center of the composite grid structure. in the samemanner as previously described for the grid of FIG. 7.

What is claimed is: 1. In a gridded electron gun: thermionic cathodeemitter means having a concave cathode emitting surface for providing acopious supply of electrons; anode electrode means spaced from saidconcave cathode emitting surface and having a central aperture in axialalignment with said concave cathode emitting surface for drawing a beamof electrons from said cathode through said central aperture in saidanode;

concave control grid means interposed between said concave cathodeemitting surface and said anode means, said concave control grid meansincluding a plurality of beam passageways therethrough of generallyuniform cross-sectional area defining electrostatic focusing lenses forfocusing said electrons into individual beamlets passable therethrough,said individual focusing lenses of said control grid increasing inlength from the outer periphery toward the center of said control grid,said control grid means being operative at a varying control gridpotential relative to said cathode emitter means.

2. The apparatus of claim 1 wherein said control grid means includesfirst and second axially spaced concave apertured control grids, saidcontrol grids being disposed adjacent each other with their respectiveapertures in axial registration, means for interconnecting said firstand second control grids to be operated at the same control gridpotential, and wherein the axial spacing between said first and secondcontrol grids increases from the outer periphery toward the center ofsaid control grids to provide the increase in length for 'the individualelectrostatic lenses formed by the respective beam passageways throughsaid aligned apertures in said control grids.

3. The apparatus of claim 1 wherein said central aperture in said anodemeans has a lesser transverse cross-sectional area than said emittingsurface of said cathode emitter for producing a convergent flow ofelectrons from said cathode emitter through said central aperture insaid anode electrode means.

4. The apparatus of claim 1 including, concave shadow grid meansinterposed in the space between said control grid means and said concavecathode emitting surface of said cathode emitter means, said shadowmeans having beam apertures in alignment with the respective beamapertures in said control grid means, and

means for operating said shadow grid means at the same potential as saidcathode emitter means.

5. The apparatus of claim 4 wherein said concave cathode emittingsurface of said concave cathode emitter means is constituted of aplurality of dimpled regions in alignment along the beam path withrespective beam passageways in said shadow and control grid means, saiddimpled regions being curved in mutually orthogonal directions andhaving radii of curvature substantially less than that of said compositeconcave cathode emitting surface.

6. In a gridded electron gun:

thermionic cathode emitter means having a concave cathode emittingsurface for providing a copious supply of electrons;

anode electrode means spaced from said concave cathode emitting surfaceand having a central aper ture in axial alignment with said concavecathode emitting surface for drawing a beam of electrons from saidcathode through said central aperture in said anode;

concave control grid means interposed between said concave cathodeemitting surface and said anode means, said concave control grid meansincluding a plurality of beam passageways therethrough of generallyuniform cross-sectional area,

said control grid means including first and second axially spacedconcave apertured control grid portions, said control grid portionsbeing disposed adjacent each other with their respective apertures inaxial registration, means for interconnecting said first and secondcontrol grid portions to be operated at the same control grid potential,whereby the second control grid portion closest to said anode isthermally shielded from the cathode by said first controlgrid portion toreduce thermionic emission from said composite control grid structure toreduce undesired interpulse noise.

1. In a gridded electron gun: thermionic cathode emitter means having aconcave cathode emitting surface for providing a copious supply ofelectrons; anode electrode means spaced from said concave cathodeemitting surface and having a central aperture in axial alignment withsaid concave cathode emitting surface for drawing a beam of electronsfrom said cathode through said central aperture in said anode; concavecontrol grid means interposed between said concave cathode emittingsurface and said anode means, said concave control grid means includinga plurality of beam passageways therethrough of generally uniformcross-sectional area defining electrostatic focusing lenses for focusingsaid electrons into individual beamlets passable therethrough, saidindividual focusing lenses of said control grid increasing in lengthfrom the outer periphery toward the center of said control grid, saidcontrol grid means being operative at a varying control grid potentialrelative to said cathode emitter means.
 2. The apparatus of claim 1wherein said control grid means includes first and second axially spacedconcave apertured control grids, said control grids being disposedadjacent each other with their respective apertures in axialregistration, means for interconnecting said first and second controlgrids to be operated at the same control grid potential, and wherein theaxial spacing between said first and second control grids increases fromthe outer periphery toward the center of said control grids to providethe increase in length for the individual electrostatic lenses formed bythe respective beam passageways through said aligned apertures in saidcontrol grids.
 3. The apparatus of claim 1 wherein said central aperturein said anode means has a lesser transverse cross-sectional area thansaid emitting surface of said cathode emitter for producing a convergentflow of electrons from said cathode emitter through said centralaperture in said anode electrode means.
 4. The apparatus of claim 1including, concave shadow grid means interposed in the space betweensaid control grid means and said concave cathode emitting surface ofsaid cathode emitter means, said shadow means having beam apertures inalignment with the respective beam apertures in said control grid means,and means for operating said shadow grid means at the same potential assaid cathode emitter means.
 5. The apparatus of claim 4 wherein saidconcavE cathode emitting surface of said concave cathode emitter meansis constituted of a plurality of dimpled regions in alignment along thebeam path with respective beam passageways in said shadow and controlgrid means, said dimpled regions being curved in mutually orthogonaldirections and having radii of curvature substantially less than that ofsaid composite concave cathode emitting surface.
 6. In a griddedelectron gun: thermionic cathode emitter means having a concave cathodeemitting surface for providing a copious supply of electrons; anodeelectrode means spaced from said concave cathode emitting surface andhaving a central aperture in axial alignment with said concave cathodeemitting surface for drawing a beam of electrons from said cathodethrough said central aperture in said anode; concave control grid meansinterposed between said concave cathode emitting surface and said anodemeans, said concave control grid means including a plurality of beampassageways therethrough of generally uniform cross-sectional area, saidcontrol grid means including first and second axially spaced concaveapertured control grid portions, said control grid portions beingdisposed adjacent each other with their respective apertures in axialregistration, means for interconnecting said first and second controlgrid portions to be operated at the same control grid potential, wherebythe second control grid portion closest to said anode is thermallyshielded from the cathode by said first control grid portion to reducethermionic emission from said composite control grid structure to reduceundesired interpulse noise.